Many times a variable speed motor drive is connected to an ac electrical power source through an ac-to-dc converting diode bridge or an individual diode rectifier and a filter formed by a capacitor and perhaps an inductor. The ac-to-dc converting rectifier receives a discontinuous quasi-sinusoidal current with high peaks and conveys a non-sinusoidal current to the load side of the rectifier. The discontinuity and non-sinusoidal character of the input ac current has four effects. First, the instantaneous current rating of the rectifier must be increased due to the high peaks and discontinuous conduction.
Second, the Fourier spectrum of the input current has a fundamental frequency component at the supply frequency and other components at multiples of twice the supply frequency, such as 2, 4, 6, 8, and 10 times the supply frequency. These non-fundamental frequency components have to be filtered so that they do not circulate a current back to the ac supply (utility supply) input. A large filter is required for this filtering and is mandated in some countries, thus increasing the cost of a load apparatus.
Third, the phase angle between the input ac voltage and the fundamental component of the current drawn by the rectifier is increased. The cosine of this phase angle is known as the power factor in power systems; a power factor that is other than unity results in under utilization of the utility supply since it causes a high reactive power component to be drawn. Home owners and small commercial establishments only pay for real power consumption, the reactive power consumption necessitated by the rectifier in the ac-to-dc rectification is not paid for by these clients. As a result, the utility must absorb the cost of the reactive power consumption, thereby suffering a monetary loss. Both the utility and users will gain if the entire generating capacity of the utility is completely used in real power consumption in the loads with zero reactive power consumption.
Fourth, when the root mean square (rms) amplitude of the electrical power source's ac voltage diminishes, due to overloading or other causes, the rms amplitude of the rectified dc voltage also changes. For a power electronic converter that receives this rectified dc voltage so as to supply a variable voltage or current and perhaps a variable frequency to a motor, the diminished rms amplitude of the input voltage negatively impacts the performance of the motor by decreasing the speed at which a full load torque can be produced and reducing the motor's capability to provide the torque and speed for which it is rated.
Consider a related art switched reluctance motor (SRM) drive system having a power electronic converter connected to an ac power source by a diode bridge rectifier. This related art system has three major disadvantages. First, when the rms voltage amplitude of the ac power source is reduced, as often occurs in many unregulated utility systems, the bridge rectifier's output dc rms voltage amplitude is diminished, thereby limiting the operational capability of the motor. Second, the current drawn by the bridge rectifier is non-sinusoidal, which induces noise and harmonics onto the utility's power lines. Third, the power factor of the system presented to the utility is less than unity, thereby negatively impacting the utility by not making use of its entire power generation for real power consumption.
FIG. 1 illustrates a related art power converter 100 for a two-phase motor. Power converter 100 includes a bridge rectifier 102 that receives ac power from an ac power source 104 and rectifies the voltage of the ac power to produce a dc power source whose energy is stored by a capacitor 106. A phase A transistor 108 regulates the flow of energy from storage capacitor 106 through a phase A winding 110 of the motor. Energy not used by the motor that is discharged by phase A winding 110 is stored within a capacitor 112 via a circuit completed by a diode 114. A phase B transistor 116 regulates the flow of energy from storage capacitor 112 through a phase B winding 118 of the motor. Energy not used by the motor that is discharged by phase B winding 118 is conveyed to and stored within capacitor 106 via a circuit completed by a diode 120.
A machine side 122 of power converter 100 has one transistor, one capacitor, and one diode per machine phase, instead of two transistors and two diodes per phase as does an asymmetric converter. Also, power converter 100 has no capacitor in between its two phases. Thus, in accordance with the operation of power converter 100 described above, a reduction of the input ac rms voltage reduces the dc rms voltage available to the motor phases, resulting in a diminished operational capability of the motor.
All reference material cited herein is hereby incorporated into this disclosure by reference.